@mixin flex_end{
    display: flex;
    justify-content: flex-end;
}
.box1{
    width: 400px;
    height: 400px;
    background: yellow;
    @include flex_end;
    position: relative;
    .box2{
        width: 300px;
        height: 300px;
        background: pink;
        @include flex_end;
        .box3{
            width: 200px;
            height: 200px;
            background: blue;
        }
    }
}